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Color‐Based Optical Detection of Glass Transitions on Microsecond Timescales Enabled by Exciplex Dynamics
摘要： Every measurement technique operates on a given timescale and measurements using emissive small molecule sensors are no exception. A family of luminescent sensors providing first optical characterization of dynamic phenomena in polymers at a timescale of several microseconds is described. This performance originates from the dynamics manifested in the excited state of the sensor molecules where diffusioncontrolled events select the emission color while radiative phenomena define the global operation timescale. Since the mechanism responsible for signal generation is confined to the short lived excited state of emissive probe, it is possible observe an unprecedented link between the timescale of sensory action and that of photoluminescence. An application of this new methodology is demonstrated by performing general, short timescale detection of glass transitions in a temperature ranges precluding the informative range of conventional techniques by tens of degrees.
关键词： glass transition,copper,luminescent sensors,responsive polymers
更新于2025-09-11 14:15:04
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Crystallization, glass transition, and molecular dynamics in PDMS of low molecular weights: A calorimetric and dielectric study
摘要： In this work linear polydimethylsiloxane (PDMS) of five low molecular weights, Wm ～2–8 kg/mol, i.e. mainly below the entanglement threshold, is studied employing differential scanning calorimetry (DSC), thermally stimulated depolarization currents (TSDC), a special dielectric technique in the temperature domain, and broadband dielectric spectroscopy (BDS). It is shown mainly via raw data that the crystallization and glass transition for the larger Wms follow expected trends, however, for Wm ～2 k (corresponding to ～22 monomers/chain) crystallization is severely suppressed and slower. Moreover, it is demonstrated that upon isothermal annealing the crystalline fraction (CF) is maximized for all Wms with the exception of the lower one. For the latter, results indicate that during the annealing only the first stages of crystallization (nucleation, ordering of the polymer around the nuclei) take place, while the main crystallization event (formation of crystals) is recorded only upon heating, via the cold crystallization process. The suppressed crystallizability originates from the retarded nucleation. In particular for the lower Wm, the presence of large numbers of chain ends should be at the origins of hindered short range ordering of the polymer chains, as well as in relation to the difficult formation of physical crosslinks. At the same time, in TSDC and BDS uniquely for Wm ～2 k upon annealing and CF～0, next to the relaxation peaks related with the glass transition of the bulk-like polymer (α and αc), an additional quite strong peak (αnew) is recorded at higher temperatures. The overall characteristics of αnew relaxation resemble those of PDMS chains constrained and ordered in the interfacial layer in amorphous silica/PDMS nanocomposites (αint relaxation). Thus, combing the overall results with results from the literature, it is proposed that αnew is a direct recoding of retarded dynamics of the polymer chains being ordered around the crystallization nuclei or around extremely small crystals.
关键词： Polymer chains ordering,Polymer crystallization,Glass transition,Polydimethylsiloxane,Thermally stimulated depolarization currents,Differential scanning calorimetry
更新于2025-09-10 09:29:36
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Photocured characteristics of fast photocurable acrylic formulations and investigations by differential photo calorimeter
摘要： Bisphenol-A glycerolate diacrylate has been synthesized, characterized and formulated with acrylate monomers. Photocrosslinking characteristics of UV-curable acrylate monomers were investigated with the help of differential photocalorimeter equipped with UV radiometer using UV radiation of 365 nm at an intensity of 10 mWcm-2. The heat of photoreaction, curing rate and degree of conversion were studied using theoretical kinetic equations to investigate the effect of 2-ethylhexyl acrylate on photocuring behaviour of bisphenol-A glycerolate diacrylate, acrylic acid and methyl methacrylate. Curing rate reached its maximum value within 7.5 s, and more than 70% monomers were photopolymerized in all the formulations. Enthalpy of photopolymerization was found in the range of - 206 to - 171 J g-1, and maximum degree of conversion attained was 82–84% within a time span of 25 s. A comparison of DPC curves reveals that 2-ethyl hexyl acrylate has no significant effect on the curing rate of photopolymerization, however, a small decrease in degree of conversion was observed. Further, FTIR quantification of C=C bond conversion was performed in order to estimate the degree of conversion after 25 s. Specific gravity and mechanical properties of the casted films have also been reported. Glass transition temperatures were also recorded with the help of DSC, and a remarkable shift from 40.52 to 23.48 °C was observed imparting flexibility to the UV-cured compositions. The developed photocurable acrylic formulations have been used for preparation of laminated glasses which were analysed for fracture and adhesion test.
关键词： 2-Ethyl hexyl acrylate,Photopolymerization,DSC,UV curing,Glass transition,Laminated glass
更新于2025-09-09 09:28:46
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Thermal behaviour of poly(methyl methacrylate)/fullerene composite films
摘要： Poly(methyl methacrylate)/fullerene composite films were fabricated by casting from toluene solutions. The mass fraction of fullerene (C60) was varied from 0.05–3 wt. %. The effect of the fullerene (C60) content on the thermal degradation parameters of the composite films was evaluated by thermogravimetric (TG) analysis. It was found that the incorporation of C60 improved the thermal stability of the polymers. The films eventually decomposed in three stages. Incorporation of fullerene caused a change in the distribution of mass loss over the stages of degradation in comparison with the pristine polymer. Using differential scanning calorimetry (DSC), the phase transitions from the glassy state to the elastic one was studied for the examined polymeric materials. New data relating to the effect of C60 on the glass transition temperature of composites with low weight fractions of filler were obtained. Specifically, for films containing up to 0.1 wt. % of C60, a single glass transition temperature was found, whereas for composites with a higher concentration of filler, two glass transition temperatures were recorded.
关键词： Composite materials,thermogravimetric (TG) analysis,glass transition,DSC,fullerenes,thermal stability
更新于2025-09-09 09:28:46
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A review on low-molar-mass carbazole- based derivatives for organic light emitting diodes
摘要： Organic light emitting diodes (OLEDs) have attracted a great deal of attention within academia and industry because of their potential applications in flat panel displays and solid-state lighting technologies. Among all low-molar-mass derivatives under investigation in OLEDs field, carbazole-based materials have been studied at length for their interesting physical properties, including good charge injection and transport, electro-luminescence, improved thermal and morphological stabilities as well as film forming properties. In addition, the relatively high triplet energy level of some substituted carbazoles makes them suitable candidates to design hosts for wide bandgap triplet emitters such as blue dopants. The article is written from structural organic chemist's point of view and is divided in several parts: V-shaped 3(2)-substituted carbazoles and derivatives with 9-carbazolyl rings, star-shaped 3,6(2,7)-substituted carbazoles and branched twin derivatives containing (di)arylcarbazolyl fragments.
关键词： Amorphous material,Ionization potential,Glass transition temperature,Organic light emitting diode,Substituted carbazole,Hole transporting material
更新于2025-09-09 09:28:46
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Medium range order and configurational entropy of vitreous silica
摘要： Two potential sets and three thermodynamic regimes were utilized in molecular dynamics simulations in the preparation of the corresponding structures of vitreous silica. Medium range order characterized by means of basic ring distributions is robust to both thermodynamic conditions and/or potentials sets. A strong correlation between topology and geometry is demonstrated; the correlation is much stronger for simple clusters/small rings. Configurational entropy was evaluated by the ideal mixing of the structural units of the decomposed silica network. Entropy responds to alterations in the heating/cooling rates and its behaviour yields an excellent agreement with the experimental specific heat capacity drops observed at the glass transition. It is suggested the glass transition of the simulated glass should be identified with the change of entropy and not with a change of enthalpy as is conventionally done.
关键词： medium range order,configurational entropy,vitreous silica,glass transition,molecular dynamics simulations
更新于2025-09-04 15:30:14
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Quartz-enhanced conductance spectroscopy for nanomechanical analysis of polymer wire
摘要： Quartz-enhanced conductance spectroscopy is developed as an analytical tool to investigate dynamic nanomechanical behaviors of polymer wires, in order to determine the glass transition temperature (Tg). A polymethyl methacrylate (PMMA) microwire with a diameter of 10 lm was bridged across the prongs of a quartz tuning fork (QTF). With the advantage of QTF self-sensing as compared with micro-cantilevers or other resonators, the resonance frequency and Q factor can be directly determined by means of its electrical conductance spectra with respect to the frequency of the external excitation source (dI/dV vs f), and therefore, no optical beam is required. The Tg of the PMMA microwire was determined by the maximum loss modulus of the QTF, calculated from the resonance frequency and the Q factor as a function of temperature. The measured Tg of the PMMA is 103 (cid:2)C with an error of 62 (cid:2)C. Both heating/cooling and physical aging experiments were carried out, demonstrating that the technique is both reversible and reproducible.
关键词： PMMA,polymer wire,quartz tuning fork,nanomechanical analysis,glass transition temperature,Quartz-enhanced conductance spectroscopy
更新于2025-09-04 15:30:14
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Glass transition temperature of PMMA/modified alumina nanocomposite: molecular dynamic study
摘要： In this study, the effect of alumina and modi?ed alumina nanoparticles in a PMMA/alumina nanocomposite was investigated. To attain this goal, the glass transition behavior of poly methyl methacrylate (PMMA), PMMA/alumina and PMMA/functionalized alumina nanocomposites were investigated by molecular dynamic simulations (MD). All the MD simulations were performed using the Materials Studio 6.0 software package of Accelrys. To obtain the glass transition temperature, the variation of density versus temperature was obtained. The temperature at which the slope of the density-temperature curve observably changes is de?ned as the glass transition temperature (Tg). The effect of alumina nanoparticles on the Tg was related to the free volume and the mobility of chain segments and the interaction between the alumina nanoparticles and the polymer. The mobility of the chain segments was investigated based on the mean square displacement and radius gyration. The results show that the incensement the Tg of the PMMA/functionalized alumina nanocomposite is more than that of the PMMA/alumina nanocomposite due to the modi?cation of the alumina nanoparticles.
关键词： molecular dynamic,nanocomposite,glass transition temperature,gyration radius
更新于2025-09-04 15:30:14